Cursed Technique: Skeleton Key Technique

Concept: The ability to unlock. This cursed technique allows the user to use their cursed energy to unlock things, without the need for its key.

Lapse: Unlock The user channels their cursed energy while either touching or very close to a lock, and performs a gesture somehow related to the nature of the lock, causing it to unlock.

Reversal: Lock By instead channeling positive energy produced with reverse cursed technique, the user reverses the effect and instead locks a lock.

Applications

Lock/Unlock: The simplest possible interpretation of this technique, the user makes a gesture reminiscent to turning a key in front of a lock, unlocking or locking it. Minimal expenditure, very easy to perform. This technique can also be performed on a door without a visible keyhole, as long as the door has a physical lock the user is aware of, though CE expenditure is increased.

Digital Bypass: A password is a key, and that which takes one is a lock. The user targets something that can be activated or unlocked with a digital equivalent of a key, such as a password or thumbprint, and gives an input. The targeted system then accepts the input as correct, regardless of what you inputted. More difficult than Lock/Unlock, but not by a lot.

Digital Shutdown: The user touches an object or interface in which something is logged in and "locks" it, logging out and/or disconnecting anyone using it. For example, using it on a screen where someone is logged in on a website would log them out, while using it on a WiFi router would disconnect everyone using it.

Cursed Unsealing: Some cursed objects and cursed tools can lock, trap or seal things, which make them locks. The user touches a cursed object or cursed tool that is in a "locked" state and speaks the word "Open". The user then expends an amount of cursed energy dependent on the tool or object, which then opens. The higher the grade of object it is, the more skill and cursed energy is required to unlock it.

Example Extension Techniques

Unlock Barrier: Barrier techniques inhibit or outright prevent passage, making them locked, which means they have a lock. The user presses their hand against the edge of a barrier, and can then either "cut" a temporary hole in the barrier and pass through, or rip down the entire barrier. Difficulty and cursed energy expenditure scale with the strength and complexity of the barrier, with the latter application being a lot more expensive than the former. More esoteric than base applications, but on the simpler end of extension techniques.

Technique Sealing: Innate cursed techniques work by channeling cursed energy through a channel in the brain, which can be locked. The user places their hand on the target's head, blocking their ability to use their innate cursed technique. Unlike other locking applications, Technique Sealing remains after use, preventing innate technique usage for a duration. This duration increases the longer the technique is maintained, allowing it to remain active for up to a few days with enough use. Can be resisted with Cursed Energy Reinforcement.

Cursed Blockage: Cursed energy also passes through the body, which can also be locked. By concentrating cursed energy into their fingertips and poking a target, they can block cursed energy passing there for a few seconds. This prevents cursed energy from reaching spots that need to pass there to do so, and blocks Cursed Energy Reinforcement in those areas. Can be resisted with Cursed Energy Reinforcement, but requires more cursed energy to do so.

Stasis: When a thing is incapable of moving, it is considered "locked", therefore a preventing movement is "locking" it. The user grabs a target and "locks" it, rendering it indestructible and immovable until the user lets go of the target or willingly ends the effect. This effect can be used on objects, non-sorcerers and sorcerers, but people require you to speak the word "Stop" to target a sorcerer and can only be maintained for a few seconds against them. Can be resisted with Cursed Energy Reinforcement.

Opening Strike: A thing which is locked prevents passage, unlocking a thing means allowing passage. The user channels cursed energy into a weapon, concentrating it at the tip, and stabs it at a target. On hit, the cursed energy is expended to "unlock" the target, pushing the material to the side with nearly zero resistance. Cannot be resisted with Cursed Energy Reinforcement.

Break the Osseous Cage: A closed cage that cannot be opened is considered locked. The body contains a bone we refer to as a cage, and which cannot be opened, making it locked. The user places their hand on the sternum of the target, and speaks the word "Open". Cursed energy then cuts a line down their sternum, before pushing out and breaking the front of the ribcage open, ripping through skin and exposing their organs. Extreme abstraction from the base technique, extremely difficult, can be resisted with Cursed Energy Reinforcement.

Domain Expansion

Hallway of a Thousand Doors: One interpretation of the technique's Domain Expansion, Hallway of Infinite Doors places everyone inside a corridor of seemingly infinite length, reminiscent of a hotal corridor, lined with doors. Every door is locked and locks again when closed, except for the user who can effortlessly open and unlock them, though a person with skill and some time could lockpick one. Anyone who successfully opens a door can instantly choose to emerge from any other door in the corridor.

Notes

So, I wanted to make a cursed technique that looks... well, really bad on the surface, but becomes insanely strong in the right hands (aka, Sukuna's hands), so I made this. It could for sure be interpreted in more ways than these (locking veins to stop blood flow, locking trajectories to prevent interference on ranged attacks, unlocking a certain man's Limitless defenses, etc), but I'm one person and not about to list every single thing that this could do. Also, most of these are far beyond the ability of whoever would end up with this technique, and more likely to be something you'd see if Sukuna fed them his finger than something the character themselves would do.